Finite-difference methods were developed in the full generality of (REF ) in [1]}, [2]}; some early works are can be found in [3]}, [4]}. We also refer to [5]} (see also Part II of [6]}) in the purely local case. In the case of (REF ) with an additional convection term, we mention e.g. [7]}, [8]}, [9]}.
